THE HEAT COAGULATION OF MILK 



H. H. SOMMER and E. B. HART 
I* 



oUU(Nc;;h 



i. (HESiS 



(From the Department of Agricultural Chemistry, University 
OF Wisconsin, Madison) 



Reprinted from 

THE JOURNAL OF BIOLOGICAL CHEMISTRY 
Vol. XL, No. 1, November, 1919 



Reprinted from The Journal of Biological Chemistry, Vol. XL, No. 1, 1919 






%> 



\^'^THE HEAT COAGULATION OF MILK* 

(J- By H. H. SOMMER and E. B. HART. 

^^"^ {From the Department of Agricultural Chemistry, University of Wisconsiii, 

Madison.) 

(Received for publication, September 17, 1919.) 

The coagulation of milk by heat was first observed by Ham- 
mersten/ who found that it occurred at from 130-150°C. with 
different samples of milk. Since then the question has been 
studied very little, and no tenable explanation has ever been given 
for the difference in the coagulating points of milks from different 
cows. In recent years a knowledge of the factors which determine 
this difference has become very desirable, for these same factors 
undoubtedly determine whether a condensed milk will coagulate 
when it is sterilized. The coagulation of condensed milk on 
sterilizing causes serious losses in the milk-condensing industry. 

In the manufacture of condensed milk, the fresh milk is first 
pasteurized or "preheated" at from 180-210°F. for from 1 to 20 
minutes. The condensing is done under vacuum at 130-160°F. 
After the desired concentration has been attained, the milk is put 
into cans, sealed, and then sterilized at 224-240°F. for 20 to 50 
minutes. It is during the sterilizing process that the coagulation 
occurs. Because it occurs so frequently, all the condensed milk 
is placed into shaking machines to break up any loose coagulum 
that may have formed. However, frequently the coagulum is so 
firm that even after shaking the milk remains lumpy. Such a 
product is rejected by the consuming public, and thus is a loss. 

Manufacturers have sought to solve the problem by controlling 
the acidity of the milk. They have set an arbitrary standard 
such as 0.18 per cent acid (calculated as lactic acid), above which 

* Published with the permission of the Director of the Wisconsin Agri- 
cultural Experiment Station. 

1 Quoted from Iterature referred to in, Kastle, J. H., Chemistry of 
milk, Hyg. Lab., Bull. 56, 1909. 

137 



138 The Heat Coagulation of Milk 

thej^ reject all milk. This has led to much difficulty, because 
often, immediately after it is drawn from the cow, milk has a 
higher titratable acidity than 0.18 per cent. Thus the con- 
denseries may be rejecting perfectly fresh milk, believing that 
they are remedying their difficulty in this way, although it has 
never been demonstrated that titratable acidity is related to the 
coagulation. The factors involved in the coagulation have never 
been determined, and no explanation is available on which to 
base a remedy for this difficulty. 

To offer an explanation for the difference in the coagulating 
points of different milk samples the following factors were studied : 
titratable acidity, hydrogen ion concentration, concentration of 
the milk, and composition and balance of the milk salts. 

The Heat Test. 

The temperature at which the milk was heated was arbitrarily 
set at ISG^C. At first the heating was done in an autoclave at 50 
pounds pressure for 20 minutes, and in that way the milks were dif- 
ferentiated into coagulating and non-coagulating. With the auto- 
clave, it took about 10 minutes to get up to the desired pressure j' 
and, after the milk had been heated, the pressure had to be released 
gradually to prevent the milk from boiling over. The disad- 
vantages of this method were such that there were no sharp limits 
from which to calculate the 20 minute interval, and it was im- 
possible to determine the relative rates at which the milk samples 
coagulated. 

To overcome these disadvantages the milk was placed into 
small glass tubes, sealed, and then heated in a xylene vapor bath 
which was constant at 136°C. within 0.5°C. The sealed tubes 
were clamped in a rack, s6 arranged that it could be tilted to invert 
the tubes, to see how the milk would flow, and in that way it was 
possible to determine the exact length of time required for each 
sample to coagulate. The milk in the sealed tubes was up to 
136°C. in less than 1 minute, so the point from which to calculate 
the time was practically the instant the tubes were inserted into 
the vapor bath. 



( -vv -V «^ -u-^ -,< 



H. H. Sommer and E. B. Hart 



139 



Titratdble Acidity. 

Since condenseries are attempting to remedy the coagulation 
problem by rejecting milk above 0.18 per cent acid (calculated as 

TABLE I. 
Titratable Acidity and Coagulation. 



May 8, 1919. 


May 10, 1919. 


May 16. 1919. 


Cow No. 


Titratable 
acidity. 


Coagu- 
lation. 


Cow No. 


Titratable 
acidity. 


Coagu- 
lation. 


Cow No. 


Titratable 
acidity. 


Coagu- 
lation. 




Lactic 
acid. 






Lactic 
acid. 




Lactic 
acid. 






per cent 


min. 




per cent 


min. 




■per cent 


min. 


1 


0.257 


20-* 


1 


0.241 


6 


31 


0.203 


4| 


2 


0.235 


3 


2 


0.231 


11 


2 


0.193 


20- 


3 


0.216 


20- 


26 


0.228 


5 


1 


0.192 


20- 


4 


0.214 


20- 


31 


0.222 


^ 


3 


0.188 


4| 


6 


0.210 


8 


27 


0.212 


6 


27 


0.188 


6| 


6 


0.207 


20- 


3 


0.212 


5§ 


6 


0.188 


20- 


7 


0.206 


6 


4 


0.211 


20- 


4 


0.188 


20- 


28 


0.201 


20- 


5 


0.205 


5i 


28 


0.186 


20- 


9 


0.200 


20- 


9 


0.197 


20- 


12 


0.184 


10 


10 


0.200 


20- 


28 


0.199 


20- 


26 


0.184 


3 


11 


0.195 


4.1 


6 


0.195 


20- 


5 


0.183 


6| 


12 


0.191 


20- 


29 


0.192 


5i 


9 


0.182 


20- 


13 


0.190 


20- 


10 


0.190 


20- 


14 


0.182 


9 


14 


0.189 


6i 


13 


0.186 


9 


29 


0.182. 


If 


15 


0.182 


20- 


12 


0.185 


11 


11 


0.178 


3^ 


16 


0.179 


4 


11 


0.184 


5 


13 


0.175 


4 


8 


0.174 


9 


8 


0.174 


20- 


10 


0.171 


20- 


17 


0.172 


20- 


14 


0.175 


6i 


30 


0.165 


20- 


18 


0.167 


20- 


15 


0.172 


20- 


15 


0.163 


20- 


19 


0.158 


12 


17 


0.166 


20- 


8 


0.163 


6| 


20 


0.157 


20- 


18 


0.160 


20- 


17 


0.156 


20- 


21 


0.156 


20- 


30 


0.162 


4 


18 


0.154 


20- 


22 


0.148 


3 


7 


0.162 


2 


7 


0.148 


2 


23 


0.146 


3^ 


21 


0.157 


20- 


20 


0.143 


20- 


24 


0.143 


20- 


20 


0.147 


20- 


19 


0.135 


20- 


25 


0.120 


2 


23 


0.145 


51 


22 


0.133 


2 








19 


0.144 


20- 


23 


0.130 


4 








22 


O.IM 


2 


24 


0.128 


20- 








24 


0.141 


20 


21 


0.128 


20- 








25 


0.131 


H 


25 


0.102 


U 



"20— = no coagulation in 20 minutes. 



140 



The Heat Coagulation of Milk 



lactic acid), it was of interest to know how much variation there 
was in the tritratable acidity of milk from individuaf cows, and 
what relation the acidity would bear to the coagulation. To study 
this, samples were taken from the University herd and titrated 
immediately, and the heat test in the xylene vapor bath applied 
as soon as possible. The results given in Table I were obtained. 

The titratable acidity varies from 0.102 to 0.257 per cent. Out 
of the 86 samples, 45 are above 0.18 per cent. 

In fresh milk there is no direct relation between titratable 
acidity and coagulation, as is evident from Table II. If fresh 
milk samples were more nearly alike in titratable acidity, then 
titratable acidity might bear a direct relationship to the heat 

TABLE II. 
Summary of Titratable Acidity and Coagulation. 



Date. 


No. of 
samples. 


No. above 

0.18 per cent 

acid.* 


No. that are 
20+.t 


No below 

0.18 pet- cent 

acid.t 


No. that are 
20+. 


May 8 

" 10 

" 16 


26 
30 
30 


15 
14 
16 


5 

7 
11 


11 
16 
14 


6 

7 
6 


Total 


86 


45 


23 


41 


19 







* Per cent above 0.18 per cent acidity, coagulating 20+ = 51.2 per cent. 
t 20+ means coagulation within 20 minutes. 

t Per cent below 0.18 per cent acidity, coagulating 20+ = 46.4 per 
cent. 

coagulation of commercial milk samples. The acidity would then 
be a measure of the amount of fermentation that had taken place. 
Lactic acid fermentation lowers the coagulating point in two ways; 
(1) it changes the reaction, and (2) it lowers the citric acid content 
of the milk very rapidly.^ Both of these are factors in lowering 
the coagulating point, as will be shown later. 

Since fresh milk samples vary so widely in titratable acidity, 
it is impossible to measure the extent of acid fermentation in a 
sample by titration. For this reason it is impossible to use titra- 
table acidity as a criterion of coagulability. 



2 Bosworth, A. W., and Prucha, M. J., Tech. Bull. 14, N. Y. Agric. Exp. 
Station, 1910. 



TABLE III. 
Hydrogen Ion Concentration and Coagulation. 



Date. 


Cow. 


pH 


Ch 


Coagulation in 
20 min. 


1919 










Feb. 21 


31 


6.55 


2.82X10-^ 


+ + + * 




32 


6.83 


1. 48X10-' 


+ 




13 


6.58 


2. 63X10-' 


+ + + 




24 


6.83 


1. 48X10-' 


— 


" 25 • 


13 


6.25 


5. 62X10-' 







32 


6.66 


2.19X10-' 


— 




31 


6.66 


2. 19X10-' 


+ + + 


" 26 


13 


6.58 


2. 63X10-' 


+ + + 




32 


6.69 


2. 04X10-' 


+ + 




31 


6.70 


1. 99X10-' 


+ + 




24 


6.73 


1. 86X10-' 


+ 




14 


6.70 


1. 99X10-' 


+ + + 


" 27 


13 


6.44 


3. 62X10-' 


+ 




32 


6.64 


2. 29X10-' 


+ + 




31 


6.64 


2. 29X10-' 


+ + + 


" 27 


24 


6.70 


1. 99X10-' 







14 


6.44 


3. 62X10-' 


- 




33 


6.59 


2. 56X10-' 


+ + + 


" 28 


13 


6.50 


3. 16X10-' 


_ 




31 


6.94 


1. 15X10-' 


+ + + 




32 


6.92 


1. 20X10-' 


- 




24 


6.93 


1.17X10- 


+ + 




14 


6.50 


3. 16X10-' 


- 




33 


6.68 


2. 09X10-' 


+ + + 


Mar. 3 


13 


6.67 


2. 14X10-' 







31 


6.84 


1. 44X10-' 


- 




33 


6.69 


2. 04X10-' 


+ + 


" 5 


33 


6.64 


2. 29X10-' 


+ + + 




13 


6.64 


2. 29X10-' 


- 




31 


6.93 


1. 17X10-' 


+ 




32 


6.79 


1. 62X10-' 


- 




24 


6.79 


1. 62X10-' 


- 




14 


6.58 


2. 63X10-' 


— 


" 6 


24 


6.97 


1. 07X10-' 


+ 




33 


6.59 


2. 57X10-' 


+ + + 


" 7 


31 


6.85 


1. 41X10-' 


+ + + 


" 10 


33 


6.79 


1. 62X10-' 


+ + + 



* Number of plus signs indicates degree of firmness. 

141 



142 The Heat Coagulation of Milk 

Hydrogen Ion Concentration. 

• 

Titratable acidity does not give an index to true acidity, or 
hydrogen ion concentration, so that if there is any relation between 
acidity and coagulation, it would be most likely to exist between 
the hydrogen ion concentration and coagulation. To study this 
possibility the hydrogen ion concentration of fresh milk was 
determined by means of the gas chain method, and the heat test 
was applied by means of the autoclave. The results given in Table 
III were obtained. 

From a study of the data it becomes evident that the hydrogen 
ion concentration is not the determining factor in the coagulation. 
Samples of equal Ch do not always respond alike to the heat test; 
one may remain liquid, and the other may form a firm coagulum. 
In a large number of cases samples of high Ch did not coagulate, 
whereas samples of lower Ch did, the exact reverse of what should 
happen if true acidity was the cause of the coagulation. 

We must conclude from this that in fresh milk Ch is not the 
determining factor in the coagulation. However, it may become 
a factor, for if we change the reaction of a milk sample by adding 
small amounts of acids the coagulating point is lowered. 

Concentration. 

The concentration of the milk would be expected to influence 
the coagulating point. This was found to be the case when milk 
was diluted (Table IV). 

TABLE IV. 

Relation, of Coagulation to Concentration. 



25 cc. of milk + H2O. 


Coagulation time. 


H2O added. 




cc. 


min. 


0.0 


H 


10 


2 


2.0 


2f 


3.0 


14 


4.0 


35- 


5.0 


35- 


6.0 


35- 



H. H. Sommer and E. B. Hart 



143 



Not only the concentration of the casein influences the coagu- 
lating point, but also the concentration of the serum. This was 
determined by comparing the effect of water dilution to the effect 
of dilution with milk serum obtained by filtering the milk through 
Pasteur-Chamberlain filters (Table V). 

In the dilution with water, where the casein and the serum are 
both diluted, the effect is greater than where the casein alone is 
diluted by adding serum; therefore, the concentration of the serum 
is also a factor influencing the coagulating point. 

Concentration of casein and of serum may in part explain the, 
difference in the coagulating points of different milk samples. 

TABLE V. 
Relation of Coagulation to Concentration of Serum. 



25 cc. of milk + serum. 


Coagulation time. 




Serum added. 






cc. 


min. 




0.0 


n 




0.1 


n 




0.2 


2 




25 cc. of milk + H2O. 


Coagulation time. 




H2O added. 




cc. 


mm. 




0.0 


n 




0.1 


2i 




0.2 


4 





However, in most cases, with the slight variation in concentration, 
this factdr is of minor importance, just as Cg is. There must be 
another factor of greater importance. 



Composition and Balance of Milk Salts. 

Since electrolytes have a very marked effect upon the stability 
of colloids, we should expect that variations in the salt compo- 
sition would influence the stability of the casein in the milk. 

That the various salts exert an influence on the coagulating 
point was shown in a number of cases. 



144 The Heat Coagulation of Milk 

The effect of an addition of ammonium oxalate to milk that 
previously coagulated is shown in Table VI. 

The removal of calcium by precipitation prevents coagulation 
in most cases and similarly in most cases the addition of small 
amounts of calcium salts lowers the coagulating point. This 
coagulation can again be balanced by means of sodium citrate or 
dipotassium phosphate (Tables VII, VIII, IX, and X). Coagu- 
lation caused by MgCl2 or BaCl2 can also be balanced by sodium 
citrate (Tables XI and XII). 

In most cases coagulation can be prevented by the addition of 
citrates or phosphates, the coagulation being due to an excess of 
calcium and magnesium. However, in a few cases the addition 
of citrates or phosphates did not prevent coagulation, but rather 

TABLE VI. 

Ammonium Oxalate Prevents Coagulation. 



5 cc. of milk + 10 per cent (NH4)2 C2O4. 


Coagulation in 20 min. 


(NH4)2 C2O4 added. 




drops 



1 

2 
3 

4 


+ + + 
+ + 
+ 



hastened it. In these cases the addition of the proper amount of 
calcium salts prevents coagulation or at least raises the coagulating 
point (Tables XIII and XIV). 

From the data we see that the calcium and magnesium are 
balanced by the phosphates and citrates of the milk practicallj^ 
in gram-equivalent amounts. The sodium and potassium chlorides 
in the concentrations present do not have any marked influence on 
the coagulating point, so that the balance of the four constituents, 
calcium, magnesium, citrates, and phosphates, largely determines 
whether a milk will coagulate or not. If calcium and magnesium 
are in excess, the milk will coagulate on heating. If calcium and 
magnesium are properly balanced with the phosphates and citrates, 
the optimum stability obtains. If phosphates and citrates are in 
excess, coagulation will also result. 



H. H. Sommer and E. B. Hart 



145 



TABLE VII. 
Balance between Calcium and Citrates 



25 cc. of milk plus. 


Coagulation time. 


m/2 Ca acetate. 


m/2 Na citrate. 


H2O 




cc. 


cc. 


cc. 


min. 


0.0 


0.0 


1.3 


3 


0.3 


0.0 


1.0 


i 

2 


0.3 


0.1 


0.9 


2h 


0.3 


0.2 


0.8 


'A 


0.3 


0.3 


0.7 


2 


0.3 


0.4 


0.6 


n 



* The sodium citrate consisted of 25 cc. of sodium m/2 citrate plus 

3 cc. of m/2 citric acid. This solution was distinctly acid, so that the 

balancing effect could not have been due to neutralization of acidity by 

means of the sodium citrate. 

TABLE VIII. 

Balance betiveen Calcium and Cilrates.* 





25 cc. of milk plu3. 




Coagulation time. 


m/2 Ca acetate. 


m/2 Na citrate. 


H2O 




cc. 


cc. 


cc. 


min. 


0.0 


0.0 


1.6 


4 


0.4 


0.0 


1.2 


1 
2 


0.4 


0.2 


1.0 


40- 


0.4 


0.4 


0.8 


40- 


0.4 


0.6 


0.6 


2i 


0.4 


0.8 


0.4 


2 



* The sodium citrate consisted of 25 cc. of m/2 sodium citrate plus 

1 cc. of m/2 citric acid. 

TABLE IX. 

Balance between Calcium and Citrates. 



25 cc. of milk plus. 


Coagulation time. 


m/2 Ca acetate. 


m/2 Na citrate. 


H5O 




cc. 


cc. 


CC. 


min. 


0.0 


0.0 


1.8 


25- 


0.8 


0.0 


1.0 


i 


0.8 


0.4 


0.6 


1 

8 


0.8 


0.6 


0.4 


25- 


0.8 


0.8 


0.2 


25- 


0.8 


1.0 


0.0 


^ 



THE JOURNAL OP BIOLOGICAL CHEMISTRT, VOL. XL, NO. 1 



146 



The Heat Coagulation of Milk 

TABLE X. 
Balance between Calcium and Phosphates. 



25 cc. of milk plus. 


Coagulation time. 


m/2 Ca acetate. 


m/2 K2HPO4 


H2O 




cc. 


cc. 


CO. 


min. 


0.0 


0.0 


1.2 


20- 


0.5 


0.0 


0.7 


1 
4 


0.5 


0.2 


0.5 


3 

8 


0.5 


0.3 


0.4 


1 


0.5 


0.4 


0.3 


20- 


0.5 


0.5 


0.2 


20- 


0.5 


O.G 


0.1 


20- 


0.5 


0.7 


0.0 


6 



TABLE XL 

Balance between Magnesium and Citrates. 





25 cc. of milk plus. 






M/2 MgCh 


m/2 Na citrate. 


H2O 




cc. 


cc. 


cc. 


min. 


0.0 


0.0 


0.7 


20- 


0.3 


0.0 


0.4 


1 
2 


0.3 


0.2 


0.2 


20- 


0.3 


0.3 


0.1 


20- 


0.3 


0.4 


0.0 


8 



TABLE XII. 

Balance between Barium and Citrates. 



25 cc. of milk plus. 




m/2 BaCli 


m/2 Na citrate. 


H2O 




cc. 

0.0 
(t.'J 
0.2 


cc. 
0.0 
0.0 
0.2 


cc. 
0.4 
0.2 
0.0 


min. 

20- 

1 

8 

20- 



Thus ilie coagulation of a milk sample on heating may be due 
either to an excess or a deficiency of calcium and magnesium. 
We may explain this in the following manner. The casein of the 
milk is most stahk' with regard to heat coagulation when it is in 
combination with a definite amount of calcium. If the calcium 



H. H. Sommer and E. B. Hart 



147 



combined with the casein is above or below this optimum, the 
casein is not in its most stable condition. The calcium in the milk 
distributes itself between the casein, citrates, and phosphates 
chiefly. If the milk is high in citrate and phosphate content, 
more calcium is necessary in order that the casein may retain its 
optimum calcium content after competing with the citrates and 
phosphates. If the milk is high in calcium, there may not be 

TABLE XIII. 
A Sample in Which Calcium Prevents Coagulation. 





25 cc. of milk plus. 




Coagulation time. 


m/2 Ca acetate. 


m/2 Na citrate. 


H2O 




cc. 


cc. 


cc. 


min. 


0.0 


0.0 


0.8 


n 


0.2 


0.0 


O.t) 


20- 


0.2 


0.1 


0.5 


u 


0.2 


0.2 


0.1 


1 


0.2 


0.3 


0.3 


3 

4 


0.2 


■ 0.4 


0.2 


3 
4 



TABLE XIV. 
A Sample in Which Calcium Raises the Coagulating Point. 



25 cc. of milk plus. 


Coagulation time. 


m/4 Ca acetate. 


HjO 




cc. 




cc. 


mm. 


0.0 




0.5 


If 


0.1 




0.4 


2 


0.2 




0.3 


91 

"4 


0.3 




0.2 


3 


0.4 




0.1 


6 


0.5 




0.0 


2h 



sufficient citrate and phosphate to compete with the casein to 
lower its calcium content to the optimum. In such a. case the 
addition of citrates or phosphates makes the casein more stable 
by reducing its calcium content. The magnesium functions by 
replacing the calcium in the citrates and phosphates. 

In most cases the coagulation is due to an excess of calcium and 
magnesium. It is possible to balance this excess by citrates, 



148 



The Heat Coagulation of Milk 







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H. H. Sommer and E. B. Hart 



149 



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150 The Heat Coagulation of Milk 

phosphates, carbonates, and other salts. It is also stated that 
danger of coagulation may be avoided in the actual practice of 
condensing milk by lengthening the "preheating" period, using 
higher temperatures. This may have the effect of lowering the 
soluble calcium content by precipitating part of it as insoluble 
calcium phosphate.^ 

To demonstrate the importance of the salt balance in the coagu- 
lation of milk, a number of samples were analyzed for total citric 
acid, phosphorus, calcium, and magnesium (Columns 2, 3, 4, and 
5, Table XV). To calculate the balance between citric acid and 
phosphates, and calcium and magnesium the percentages were 
converted into gram-equivalents as follows: 



(a) 


Citric acid 


100 


192 




(&) 


PjOs X 


100 


7* 


71 




11 


(c) 


CaO X 100 
56 




rJ^ 


MgO X 


100 





40 

* Multiply by tt because at pH 6.50, the average reaction of milk, the 
ratio of primary to secondary phosphate, is such that the mean basicit}"- 
of the phosphates is approximately yr of what it would be if all the 
phosphates were secondary phosphates. 

Column 10 shows the sum of citric and phosphoric acids in 
gram-equivalents; Column 11, the sum of calcium and magnesium 
in gram-equivalents. Column 12 shows the balance; a plus sign 
showing an excess of calcium and magnesium, and a minus sign 
showing an excess of citric and phosphoric acids. In only a few 
cases is there an excess of citric and phosphoric acids, and the 
excess is small. Those that coagulated had the largest excess of 
calcium and magnesium. To make this result more apparent. 
Column 13 shows the values of Column 12 minus 0.40. This figure 
was arbitrarily chosen and subtracted so as to make the coagulat- 
ing samples have a plus sign and the non-coagulating samples have 
a minus sign. In five cases out of the thirty this result does not 
hold. However, the fact that, in twentj^-five out of the thirty 
samples, those having the highest excess of calcium and magnesium 



H. H. Sommer and E. B. Hart 151 

over citrates and phosphates coagulated and those having the 
lowest excess did not coagulate, indicates that this factor is very 
important. 

The five exceptions may be due to the other factors, concen- 
tration and reaction. Samples 2 and 9, with their small excesses 
of calcium and magnesium, should not coagulate; however, both 
samples are high in total solids. Samples 20 and 26 did not coag- 
ulate although the excess of calcium and magnesium is high ; again 
the explanation may lie partly in the concentration of the milk, 
both samples being low in total solids. If the pH had been 
determined we might have gained further insight into these 
exceptions and an explanation for the irregularity of Sample 29. 

SUMMARY AND CONCLUSIONS. 

1. The main factor in the heat coagulation of fresh milk is the 
composition of the milk salts. Apparently casein requires a 
definite optimum calcium content for its maximum stability. 
The calcium content of casein is largely controlled by the mag- 
nesium, citrates, and phosphates present. 

2. In fresh milk there is no relation between titratable acidity 
and heat coagulation. 

3. Acid fermentation in milk lowers the coagulating point by 
changing the reaction and by lowering the citric acid content. 
However, the titratable acidity of fresh milk samples varies so 
widely that it is impossible to determine the extent of acid fer- 
mentation by titration. Therefore it is impossible to use the 
acidity of milk as a criterion of coagulability. 

4. Difference in concentration accounts partly for the difference 
in coagulation of fresh milk samples. 

5. Hydrogen ion concentration is not the determining factor in 
fresh milk coagulation. It is nevertheless a factor in fresh milks, 
and in commercial milks it may become an imoortant factor. 



LIBRARY OF CONGRESS 

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BALTIMORE. U. 8. A. 



